1. Field of the Invention
This invention generally relates to a polymeric material useful for coating an implantable device, such as a stent.
2. Description of the Background
Although stents work well mechanically, the chronic issues of restenosis and, to a lesser extent, stent thrombosis remain. Pharmacological therapy in the form of a drug-delivery stent appears a feasible means to tackle these biologically derived issues. Polymeric coatings placed onto the stent serve to act both as the drug reservoir, and to control the release of the drug. One of the commercially available polymer coated products is stents manufactured by Boston Scientific. For example, U.S. Pat. Nos. 5,869,127; 6,099,563; 6,179,817; and 6,197,051, assigned to Boston Scientific Corporation, describe various compositions for coating medical devices. These compositions provide to stents described therein an enhanced biocompatibility and may optionally include a bioactive agent. U.S. Pat. No. 6,231,590 to Scimed Life Systems, Inc., describes a coating composition, which includes a bioactive agent, a collagenous material, or a collagenous coating optionally containing or coated with other bioactive agents.
The nature of the coating polymers plays an important role in defining the surface properties of a coating. For example, a very low Tg, amorphous coating material can have unacceptable rheological behavior upon mechanical perturbation such as crimping, balloon expansion, etc. On the other hand, a high Tg, or highly crystalline coating material introduces brittle fracture in the high strain areas of the stent pattern.
A current paradigm in biomaterials is the control of protein adsorption on the implant surface. Uncontrolled protein adsorption, leading to mixed layer of partially denatured proteins, is a hallmark of current biomaterials when implanted. Such a surface presents different cell binding sites from adsorbed plasma proteins such as fibrogen and immunogloblulin G. Platelets and inflammatory cells such as monocyte/macrophages and neutrophils adhere to these surfaces. Unfavorable events can be controlled by the use of non-fouling surfaces. These are materials, which absorb little or no protein, primarily due to their hydrophilic surface properties.
Another limitation of current drug-delivery stents stems from the fact that the stent is a foreign body. Use of drug-delivery stents has proved successful by use of controlled release of anti-proliferative or anti-inflammatory drugs to control restenosis. However, drug-delivery stents still have a small, but measurable, incidence of sub-acute thrombosis. Moreover, drug-delivery stents have not driven restenosis to zero levels, especially in more challenging patient subsets such as diabetics or patients with small vessels, and/or long, diffuse lesions. A biomaterials-based strategy for further improving the outcome of drug-delivery stents is by the use of biobeneficial materials or surfaces in stent coatings. A biobeneficial material is one which enhances the biocompatibility of a device by being non-fouling, hemocompatible, actively non-thrombogenic, or anti-inflammatory, all without depending on the release of a pharmaceutically active agent.
Some of the currently used polymeric materials such as poly(vinylidene-co-hexafluoropropene) have good mechanical properties, and acceptable biocompatibility, but also have low permeability to drugs. One proposed solution to ameliorate this issue is to blend in hydrophilic polymers. However, it is well known in the art that many hydrophilic materials such as polyethylene oxide or hyaluronic acid are water-soluble and can be leached out of the composition such that the coating may lose biobeneficiality. Such polymeric blends can also have compromised mechanical properties, particularly the ultimate elongation.
The present invention addresses such problems by providing a polymeric material for coating implantable devices.